I. Field of the Invention
The present invention relates to a method for recording images or pictures by using electrostatic latent images, and particularly to a picture recording method and apparatus for obtaining a toner image by developing, without disturbance, a visualized image (toner image) formed previously on a latent image carrier.
II. Description of the Related Art
Various color image recording methods utilizing electronic photography methods have been proposed. An example of one such color picture recording method is a "repeated developing" method. The repeated developing method produces a color picture using a process whereby electrostatic latent images of two or three levels are formed on a single photosensitive medium. The first latent image of the photosensitive medium has latent images of two or three levels and is developed by a first developing device, thereafter the second latent image on the photosensitive medium is developed by a second developing device and then a finally formed toner image is transferred at a single time. This method is very effective in reducing size and obtaining a high copying speed.
However, in such a repeated developing method, the photosensitive medium carrying the toner image through the first developing process is then rubbed by the developer in the second and successive processes, and the toner image formed by the first developing process is disturbed by the later developing processes. As a result, this method is accompanied by the problem that the color picture finally obtained is considerably flawed. Therefore, there is a need for a picture forming method using a repeated developing method to develop successive images that does not disturb toner images of preceding images.
It is advantageous to develop successive images with a single-element no-contact development process in order not to disturb the toner image on the photosensitive medium. However, the single-element no-contact development method has problems with high speed operation. It is, therefore, preferable to use a double-element developer consisting of a carrier and toner.
However, in this case, if the magnetic brush developing method is used, developing is done by depositing the double-element developer on a non- magnetic sleeve having a magnetic roller therein and rubbing a latent image with a magnetic brush. Therefore, where the magnetic brush developing method is used, the toner image formed while developing the preceding image is disturbed because the toner image is rubbed with the tip of the magnetic brush while developing subsequent images.
As a means for solving such problems, Japanese Patent Application Unexamined Publication No. 126665/1985 proposes a color image developing device which uses a double-element developer, mixing a magnetic carrier having a grain size of 50 micrometers (.mu.m) or less with the toner particles. A reduction in grain size of the carrier improves the effects of disturbance of the image, but when the grain size becomes smaller, more carrier transfers to the surface of the photosensitive medium from the developing device, resulting in a distinctive carry-over phenomenon. In order to avoid the carry-over phenomenon, the magnetic force must be enhanced. Accordingly, it is necessary to make the grain size of the carrier particle large. Therefore, regulating only the carrier grain size cannot result in sufficiently satisfactory results.
Various image forming methods to easily form and record composite pictures, by utilizing electronic photography methods, have been proposed. The "repeated negative exposing method" is typical of such a method using a single developing device. In this method, after the photosensitive medium of the electronic photography device is uniformly charged, a latent image of a first picture is negatively written on the photosensitive medium by the exposing means. A latent image of second picture is also formed by the negative writing method to combine the second picture with the first picture. The first and second latent images are inverted to form the composite picture.
Representative of composite picture forming methods using two developing devices is a method to form a combined picture by charging, exposing a first negative (or positive) image, exposing a second positive (or negative) image, a first developing (regular developing or inverse developing) process, and a second developing (inverse developing or regular developing) process.
Moreover, the Japanese Patent Application Unexamined Publication No. 2047/1982 discloses a method utilizing an image forming process consisting of charging, exposing a first negative image, a first developing (inverse developing) process, exposing a second positive image, and a second developing (regular developing) process.
The repeated negative image exposing method is certainly simplified in structure but has a disadvantage that the pictures cannot be combined on an ordinary positive document.
The present invention is proposed to resolve the above problems. It is therefore an object of the present invention to provide a method of recording images which develop images without disturbing the existing toner image, even if a double-element developer is used.
It is also an object of the present invention to provide a color image recording method which uses a double-element developer and which develops images without disturbing the existing toner image.
It is also an object of the present invention to provide a picture recording method which can combine pictures to a positive document, ensure good reproducibility of low concentration pictures, eliminate disturbance of the image formed by the first developing process, and prevent picture quality from being gradually deteriorated.
An important teaching of the present invention is that the density of the carrier used in the double-element developer is an important factor relating to the disturbance of the toner image when used in a magnetic brush developing device utilizing a double-element developer.